The WISSH Quasars Project III. X-ray properties of hyper-luminous quasars

We perform a survey of the X-ray properties of 41 objects from the WISE/SDSS selected Hyper-luminous (WISSH) quasars sample, composed by 86 broad-line quasars (QSOs) with bolometric luminosity $L_{Bol}\geq 2\times 10^{47},erg, s^{-1}$,atz2-4.Allbut3QSOsshowunabsorbed2-10keVluminosities$L_{2-10}\geq10^{45} ,erg ,s^{-1}$. Thanks to their extreme radiative output across the Mid-IR-to-X-ray range, WISSH QSOs offer the opportunity to significantly extend and validate the existing relations involving $L_{2-10}$. We study $L_{2-10}$ as a function of (i) X-ray-to-Optical (X/O) flux ratio, (ii) mid-IR luminosity ($L_{MIR}$), (iii) $L_{Bol}$ as well as (iv) ${\alpha }_{OX}$ vs. the 2500$\mathring{A}$ luminosity. We find that WISSH QSOs show very low X/O(<0.1) compared to typical AGN values; $L_{2-10}\mathrm{/}{L}_{MIR}$ ratios significantly smaller than those derived for AGN with lower luminosity; large X-ray bolometric corrections $k_{\mathrm{B}\mathrm{o}\mathrm{l},\mathrm{X}}\sim$ 100-1000; and steep $-2<{\alpha }_{OX}<-1.7$. These results lead to a scenario where the X-ray emission of hyper-luminous quasars is relatively weaker compared to lower-luminosity AGN. Models predict that such an X-ray weakness can be relevant for the acceleration of powerful high-ionization emission line-driven winds, commonly detected in the UV spectra of WISSH QSOs, which can in turn perturb the X-ray corona and weaken its emission. Accordingly, hyper-luminous QSOs represent the ideal laboratory to study the link between the AGN energy output and wind acceleration. Additionally, WISSH QSOs show very large BH masses ($\log [M_{\mathrm{B}\mathrm{H}}\mathrm{/}{M}_{\odot }]$>9.5). This enables a more robust modeling of the $\mathrm{\Gamma }-M_{BH}$ relation by increasing the statistics at high masses. We derive a flatter $\mathrm{\Gamma }$ dependence than previously found over the broad range 5 <$\log (M_{\mathrm{B}\mathrm{H}}\mathrm{/}{M}_{\odot })$ < 11.